A vast "shadow inventory" of foreclosed homes that banks are holding off the market could wreak havoc with the already battered real estate sector, industry observers say.

Lenders nationwide are sitting on hundreds of thousands of foreclosed homes that they have not resold or listed for sale, according to numerous data sources. And foreclosures, which banks unload at fire-sale prices, are a major factor driving home values down.

"We believe there are in the neighborhood of 600,000 properties nationwide that banks have repossessed but not put on the market," said Rick Sharga, vice president of RealtyTrac, which compiles nationwide statistics on foreclosures. "California probably represents 80,000 of those homes. It could be disastrous if the banks suddenly flooded the market with those distressed properties. You'd have further depreciation and carnage."

In a recent study, RealtyTrac compared its database of bank-repossessed homes to MLS listings of for-sale homes in four states, including California. It found a significant disparity - only 30 percent of the foreclosures were listed for sale in the Multiple Listing Service. The remainder is known in the industry as "shadow inventory."

"There is a real danger that there is much more (foreclosure) inventory than we are measuring," said Celia Chen, director of housing economics at Moody's Economy.com in Pennsylvania. "Eventually those homes will have to be dealt with. If they're all put on the market, that will add more inventory to an already bloated market and drive down home prices even more."

More than one-third locally

In the Bay Area, a Chronicle analysis of data from San Diego's MDA DataQuick shows that more than one-third of foreclosures are in shadow territory - that is, they are not registering in county records as having been resold.

For the 26 months from January 2007 through February 2009, banks repossessed 51,602 homes and condos in the nine-county Bay Area, according to DataQuick. Yet in the same period, only 30,823 foreclosures were resold, leaving about 20,000 bank repos unaccounted for.

Turnaround usually quick

Realtors say foreclosures generally go on the market a month or two after the bank takes title and then sell fairly quickly, often getting an accepted offer within a week or two of being listed and then closing escrow within 30 days. That means that foreclosures should register as being resold within three months.

But taking the foreclosures in any given month or selection of months and looking at what happened three months later also reveals a big gap between what banks took back and what they resold.

Tom Kelly, a spokesman for banking giant Chase in Chicago, said the bank sells foreclosed homes in a timely fashion.

"We try not to be in the business of owning homes," he said. "Our goal is to get them back on the market as quickly as possible. We want to maximize what we sell them for and yet do it quickly."

Kelly was at a loss to explain the shadow inventory phenomenon other than the quantities involved.

"The inventory might be growing because there is just a lot of volume coming in. That would not surprise me," he said.

Locally, the monthly number of foreclosures has decreased since peaking at 4,321 in August 2007. That has allowed foreclosure resales to start closing the gap.

Most observers say the recent fall-off in foreclosures came because California and many banks implemented foreclosure moratoriums in the fall, not because the problem has diminished.

Only 65.5 percent resold

A second DataQuick study of all Bay Area homes repossessed by banks in the 18 months ending January 2009 tracked how many of those homes had resold by mid-March. It found that 65.5 percent had resold. Discovery Bay's ForeclosureRadar.com compared its database of Bay Area foreclosures to MLS listings for the past 120 days and found that fewer than one-fifth of the foreclosures showed up as for-sale listings.

-- The "pig in the python": Digesting all those foreclosures takes awhile. It's time-consuming to get a home vacant, clean and ready for sale. "The system is overwhelmed by the volume," Sharga said. "In a normal market, there are 160,000 (foreclosures for sale nationwide) over the course of a year. Right now, there are about 80,000 every month."

-- Accounting sleight-of-hand: Lenders could be deferring sales to put off having to acknowledge the actual extent of their loss. "With banks in the stress they're in, I don't think they're anxious to show losses in assets on their balance sheets," O'Toole said.

-- Slowing the free-fall: Banks might be strategically holding back some foreclosures so prices don't fall as fast. "They want to be careful about not releasing them too quickly so they don't drive prices down and hurt the values," O'Toole said.

Besides the shadow foreclosures, yet another wave of distressed properties is in the pipeline. These are homes with delinquent payments for which the banks appear to be prolonging the foreclosure process. Some of that could be because they're negotiating with homeowners about loan modifications or other ways to keep them in the home. But banks also could be deliberately foot-dragging for the same three reasons listed above.

"The problem is that no one knows how extensive (the shadow inventory) is," said Patrick Newport, U.S. economist with the Massachusetts research firm Global Insight. "It's a wild card. If it's a really big number, you'll see prices drop a lot more and deeper problems for the financial system."

Missing foreclosures

Only 65.5 percent of all Bay Area homes repossessed by banks in the 18 months ended January 2009 had been resold by mid-March. This study looked at the same homes over time, not an aggregate of all foreclosures.